So far, according to an electrospinning method, a nonwoven fabric, in which fibers having a uniform fiber diameter form a three-dimensional network structure, and pore diameters are uniform, can be produced.
The electrospinning method like this is a method of spinning, in which while a spinning stock solution is fed to a spinning space, an electric field is applied to the fed spinning stock solution to draw and accumulate on a counter electrode. Like this, fibers drawn and spun by the action of an electric field are accumulated on the counter electrode directly by a force of an electric field; accordingly, a paper-like nonwoven fabric is formed. However, when the nonwoven fabric is used as a heat insulating material, a filter, and the like, a bulky nonwoven fabric is preferred.
Accordingly, the present applicant has proposed a process for producing a bulky nonwoven fabric (=neutralization spinning method, see patent literatures 1 and 2), which includes a step of electrifying a spinning polymer solution, a step of supplying the electrified polymer solution into a spinning space to fly by an electrostatic force, a step of irradiating the supplied and formed fibers with ions having a polarity opposite to that of the fibers, and a step of collecting spun fibers.
Since the bulky nonwoven fabric is a down-like nonwoven fabric having low density, where fibers do not adhere to each other, or, adhere very weakly to each other; it can be used in applications where a shape-retaining property and a mechanical strength are not required. However, in the application such as liquid filtration, where the shape-retaining property and mechanical strength are necessary, in some cases, a nonwoven fabric form could not be maintained to result in inadequacy in practical use.
Therefore, as a method of imparting the shape-retaining property and mechanical strength to neutrally spun inorganic fiber nonwoven fabric, for example, methods such as shown below can be considered.
(1) A method where inorganic fibers are papermade to form an inorganic fiber nonwoven fabric
(2) A method where a binder is sprayed to an inorganic fiber nonwoven fabric and dried
(3) A method where an inorganic fiber nonwoven fabric is dipped in a binder bath, followed by passing through a two-roll press (an apparatus for applying pressure) to remove an excess binder, further followed by drying
However, according to the method of (1), it is very difficult to wet-lay nanofibers manufactured by the electrostatic spinning method. According to the method of (2), it is difficult to uniformly impart a binder over an entirety of the inorganic fiber nonwoven fabric (in particular, extend to the inside of the nonwoven fabric) to result in poor mechanical strength. According to the method of (3), gaps of the inorganic fiber nonwoven fabric are collapsed to result in incapability of maintaining the bulkiness and in low porosity.
Now, a fiber structure including the inorganic fiber nonwoven fabric like this can be used as a culture carrier. In order to culture cells in the state close to an environment in a living body, there is a tissue formation induction technique by three-dimensionally culturing cells. As the culture carriers, a film, a particle, a hollow fiber, a fiber aggregate, and a foam are well known.
However, these culture carriers are insufficient in surface area that is a scaffold of cells necessary for three-dimensional cultivation; accordingly, in many cases, high density cultivation of cells is difficult and a tissue formation function of cells, which is similar to an in vivo environment is not possessed.
As a culture carrier that can solve such problems of an existing culture carrier and can be three-dimensionally cultured, “a scaffold including nanofibers prepared by an electrospinning method” has been proposed (Patent literature 3). In specific examples, nanofibers made of silica or PVA are used.
However, even the scaffold including nanofibers like this is low in a cell proliferative potential and difficult to culture densely, in addition, also a cell function was difficult to develop. Furthermore, fiber density and thickness are difficult to control, and clusters of cells are irregularly formed; accordingly, it was difficult to observe a culture state.
Still furthermore, in order to impart a function to a fiber structure, a metal ion-containing compound is imparted. When, for example, calcium that engages with a broad range of biological reactions such as cell division, proliferation and differentiation, clotting of blood, muscle contraction, excitation of nerve sensory cells, phagocytosis, antigen recognition, immune reaction such as antibody secretion, and secretions of various kinds of hormones, and forms a hydroxyapatite crystal together with phosphorus to precipitate to matrix structures of bone and tooth to impart mechanical strength; sodium that works for maintaining osmotic pressure of extracellular liquid; iron that is an indispensable site of an electron carrier (cytochrome C) in transportation of oxygen and energy metabolism; magnesium that is an important inorganic component of bone and tooth; potassium that works for maintaining nervous excitement, muscle contraction, and maintaining osmotic pressure inside of cells; or metals such as copper, iodine, selenium, chromium, zinc, and molybdenum are imparted to a fiber structure, the fiber structure can be used as a cell culture substrate that can improve a cell function, or a fiber structure having antibiotic properties.
For example, in patent literature 3, “a scaffold including nanofibers surface-modified with a cell attachment factor (in particular, calcium phosphate) prepared by an electrospinning method” (claims 1, 8, and 9) has been proposed. Specifically, it is disclosed that silica nanofibers synthesized according to a sol-gel method is preferred, an adhesion rate of cells can be controlled by making hydrophilic or hydrophobic depending on a heat temperature of silica nanofibers, and phosphorus lime can be precipitated on a surface of nanofibers by incubating silica nanofibers in a simulated body liquid to be useful as an artificial bone cell carrier (paragraphs 0015, 0018, 0025, and the like).
However, a scaffold including nanofibers like this was low in cell function.